In such a light-emitting assembly, it may arise that, in an undesired manner, a dust or dirt particle is deposited at a point on the surface of the optical element which is in the path of the light emitted by the light-emitting element. The light impinging on this particle is generally absorbed to a considerable extent by the particle, with the result that there is a local temperature increase which is then propagated to a greater or lesser extent by thermal conduction within the optical element. If the temperature exceeds the softening point or melting point of the material of which the optical element consists, deformation or fusing of the optical element takes place. If, owing to the heating, coloring of the optical element also takes place, a “chain reaction” can moreover occur since, in general, again more radiation or light is absorbed by such a coloration. The temperature increase in question can therefore result in damage to the optical element. Furthermore, under certain circumstances, further component parts of the light-emitting assembly may also be damaged and the light-emitting assembly may even be completely destroyed.
The outlined problem accordingly results in particular when the optical element consists of a material which has a correspondingly low thermal stability, i.e. in particular a comparatively low softening point or melting point or decomposition point. In practice, this is in particular the case when the optical element consists of plastic. The optical element may be, for example, a plastic lens or a diffusing film consisting of plastic. In this case, even very low levels of contamination can result in the described effect taking place.
FIG. 3 shows part of a correspondingly destroyed light-emitting assembly. The destroyed optical element 20, in this case in the form of a destroyed plastic lens, is shown. The plastic lens is surrounded by a reflector 30, which has likewise been damaged. The component parts have been damaged or destroyed by fusing or decomposition. As mentioned above, therefore, not only destruction of the optical element, in this case the plastic lens, but also damage to further components of the light-emitting assembly can take place.
To further demonstrate the effect, FIG. 4 shows part of a corresponding light-emitting assembly in which a point 22, which simulates or represents a corresponding dust particle, has been applied to the light entry surface 21 of the plastic lens, denoted by 20′ here, with a felt tip pen. This assembly has been provided with a light-emitting element, in this case in the form of an LED (light-emitting diode) light source and has been operated for 2 hours with the light-emitting element emitting light. FIG. 5 shows the state after 2 hours: practically the entire light entry surface 21 is irregularly deformed and black as a result of the destruction.